1. FIELD OF THE INVENTION
The present invention relates to a mixer circuit arrangement used in a high frequency signal receiver, and more particularly to an improvement in the mixer circuit arrangement employing a distributed constant circuit using a strip line as a basic component thereof.
2. DESCRIPTION OF THE PRIOR ART
The mixer circuit used in the receiver comprises a frequency converting device such as a diode, a band pass filter for exclusively passing a high frequency signal to apply the same to the device, a band pass filter for exclusively passing a local oscillation signal to apply the same to the device, and a low pass filter for extracting from the device only an intermediate frequency signal of a frequency equal to the difference between the frequency of the high frequency signal and the local oscillation signal frequency. It is well known that a frequency converting device such as the diode produces, in addition to the intermediate frequency signal having a frequency equal to the frequency of the input high frequency signal and the local oscillation signal frequency, an image signal of a frequency equal to the difference between a harmonic having a frequency which is twice as high as the frequency of the local oscillation signal and the frequency of the input high frequency signal. The suppression of the occurrence of the image signal leads to the enhancement of the performance of the mixer circuit. To this end, it is necessary to provide a sharp characteristic with the band pass filter for the input high frequency signal to sufficiently suppress the local oscillation signal, and further to provide a sharp characteristic with the band pass filter for the local oscillation signal to sufficiently suppress the input high frequency signal and the image signal. The characteristic of those band pass filters should be extremely sharp particularly when the intermediate frequency is very low relative to the frequency of the input high frequency signal. Such sharp characteristics can be attained relatively easily when a microwave circuit using a wave guide as a band pass filter is employed.
On the other hand, a distributed constant circuit using a microwave strip line which is simple in circuit arrangement and which can be constructed in a small size has been known. When such a microwave strip line is used in the mixer circuit for an SHF band signal receiver, the circuit construction can be readily constructed in a small size. However, in this case, the frequency selectivity characteristics of the two band pass filters described above must be very sharp because the intermediate frequency for the SHF band is low.